The etiology and pathogenesis of mycosis fungoides (MF) and its leukemic variant, the Sezary syndrome have remained in question. While adult T cell leukemia/lymphoma (ATLL) is known to be caused by infection with the Human T Lymphotropic Virus Type I (HTLV-I), the majority of patients with MF have no antibodies and their malignant cells (Sezary cells), have not yet been shown to harbor the virus. The P.I. has succeeded in culturing the blood lymphocytes of patients with MF and has detected virus particles resembling HTLV-I in cultured cells of practically all patients. Most of these cultures have become immortalized. The particles in some specimens react with antisera to HTLV-I/II, whereas in other cultures, they are not reactive. Recently, utilizing a variety of specific primers and probes, it has been shown by PCR that the blood cells of 45/50 consecutive MF patients harbor HTLV-I sequences, whereas the cells of one patient proved to harbor a deleted proviral sequence specific for HTLV-II. Therefore, a role for HTLV or incomplete forms of these viruses in the pathogenesis of MF is entertained. This project will examine the mechanism(s) by which this retrovirus affects the evolution of the disease. Firstly, the cell(s) which harbor the virus in vivo will be identified. It is not known whether the neoplastic CD4+ cell is infected per se, or whether it is transformed indirectly. Therefore, freshly isolated specimens of MF blood, skin and lymph nodes will be fractionated to enrich for various subpopulations of lymphocytes, monocytes, dendritic and Langerhans cells. The populations found to harbor proviral sequences by PCR will then be subjected to in situ hybridization combined with histochemistry or, if the signal proves to be too weak, in situ PCR and immunoelectron microscopy will be done to identify the infected cell. Conversely, the pathogenic effect of HTLV- I/II will be studied in vitro. To this end, normal CD4-positive lymphocytes, monocytes, Langerhans, and dendritic cells will be cocultured with irradiated bona fide HTLV-infected cell lines, virus=containing MF cultures and the alleged HTLV transforming protein, tax. If the target cells become infected in vitro, the effect on their physiology (e.g. cytokine production, cytotoxicity) and parameters of transformation (immortalization, karyotypic changes and rearrangement of the T cell receptor beta chain) will be assessed. The infectivity of MF cells and/or their cultures will also be tested in rabbits. In addition, ultrastructural and freeze-fracture analyses of HTLV-I/II and their effect on mammalian cell membranes during viral uptake, proliferation, and polykaryon formation will be analyzed. These studies may help elucidate when, in its evolution, the inexorably fatal course of MF may be arrested.